1. Field of the Invention
The present invention relates generally to silicon wafer handling machines, and more particularly to an improved clean air system for a pod loader interface ("PLI") system particularly adapted to unload and reload Standard Mechanical InterFace ("SMIF") pods.
2. Description of the Prior Art
Handling of silicon wafers is critical to the IC manufacturing process. Any physical damage to the wafers will decrease the chip yield, which is a prime factor for the profitability of the semiconductor device manufacturing. Contamination of the wafer by particulates or other contaminants decreases chip yield.
The desire to shield the wafers from contaminants has led to the development and use of SMIF pods in the semiconductor processing industry. The SMIF pods allow wafers to be transported in a sealed environment, so that they are not exposed to ambient air.
After an SMIF pod reaches a wafer processing apparatus, they must be opened, and the wafer carrier inside must be placed in position for the desired process operation. If the unloading and positioning operation is performed manually the wafers are subjected to an increased risk of damage from mishandling as well as increased exposure to contamination.
Other prior art attempts to maintain a clean air environment relative to semiconductor wafer transfer and processing operations fail to achieve the advantages of the present invention.
U.S. Pat. No. 5,241,758 issued Sep. 7, 1993, to Cruz et al. ("Cruz patent") applies clean air via a solid plate with a central hole through which the clean air passes over a single disc-shaped surface of semiconductor wafers placed directly below the plate. The device of the Cruz patent is used in operations on the wafers one-at-a-time and could not be used on a pod of stacked wafers used in transferring wafers, such as the SMIF pod.
U.S. Pat. No. 5,362,274 issued Nov. 8, 1994, to Seiichiro ("Seiichiro patent") blows clean air through an arched conduit having multiple horizontal slits over wafers in an apparatus for washing semiconductor materials. The conduit device of the Seiichiro patent is relatively complex and expensive to construct and takes up a substantial amount of space due to the arched conduit. A prior art device shown in FIGS. 7 and 8 of the Seiichiro patent employs a vertical screen perforated with widely spaced holes for directing a flow of clean air horizontally across semiconductor wafers. The Seiichiro patent reports that this prior art device exhibits in an undesirable turbulent air flow.
U.S. Pat. No. 4,927,438 issued May 22, 1990, to Mears et al. ("Mears patent") provides horizontal clean air flow over wafers in a load chamber of a load lock by pumping air up from a vertical duct through a vertical filter adjacent to the wafers and also downward clean air flow through a horizontal air filter above the wafers, all in a closed air circulation within the load chamber. The Mears patent lacks any description of air flow uniformity either vertically across the vertical filter, or horizontally across the horizontal filter.
U.S. Pat. No. 4,963,069 issued Oct. 16, 1990, to Wurst et al. ("Wurst patent") describes a wafer carrier having a fan and filter on top of the carrier with clean air blowing downwardly onto the wafers which are oriented vertically with the bottom of the container open. Wurst also indicates that the fan and filter may be positioned on the side of the carrier with the clean air blowing horizontally over the wafers with the other side of the container open. A secondary clean air flow in the process apparatus in a direction parallel to that of the air flow in the container and a third clean air flow in a transfer device is transverse to the air flow in the container. The container with fan and filter is more costly and complex than a sealed SMIF pod, and should the container fan fail contaminants could enter the wafer carrier.
U.S. Pat. No. 4,904,153 issued Feb. 27, 1990, to Iwasawa et al. ("Iwasawa patent") provides a transporting robot for semiconductor wafers in which the wafers are stacked vertically in holders which are moved by robot arms while overhead fans blow downwardly through horizontal filters toward the wafers below. In the Iwasawa device the filter is separated by a substantial distance from the wafers so that the air velocity at the wafers would be reduced and would likely be turbulent rather than laminar air flow and there would not likely be an even flow over all of the wafers.
Fortrend Engineering Corporation has developed a PLI system which automatically unloads a SMIF pod and positions the wafer carrier and the wafers contained therein so as to be ready for the next process operation, and then reloads the wafer carrier and the wafers carried therein into the SMIF pod after the process step is completed. That PLI system, as described in U.S. patent application Ser. No. 08/400,039 filed Mar. 7, 1995, that is hereby incorporated by reference, includes an elevator that raises the SMIF cover away from its base to reveal the wafer carrier contained in the SMIF pod. An articulated arm thereafter reaches through an opening in a bulkhead of the machine, securing the wafer cassette and withdrawing it through the hole in the bulkhead. The wafer cassette is then placed in position to be operated upon by the next step in the manufacturing process. A loading platform that supports the wafer carrier moves up and down when activated by an electric motor to adjust the height of the wafer carrier for proper unloading to and reloading from the various devices that operate on the wafers.
The PLI system maintains the wafer carrier carrying the semiconductor wafers in an ultra-clean micro-environment by supplying ultra-clean air that flows horizontally across planar, disk-shaped surfaces of the semiconductor wafers present in the wafer carrier. To provide the flow of air, the prior art PLI system includes a horizontal portion of a plenum chamber that is located beneath the vertically movable loading platform supporting the wafer carrier. The horizontal portion of the plenum chamber communicates with a vertical portion of the plenum chamber that is displaced to one side of the loading platform and extends upward a distance that slightly exceeds the height of the SMIF pod. In this PLI system, air flowing out of the vertical portion of the plenum chamber passes in a straight line directly across the semiconductor wafers present in the wafer carrier located on the loading platform to exit the micro-environment through the opening in the PLI system's bulkhead. This direct flow of air across the semiconductor wafers and through the bulkhead opening differs from wafer handling systems that lack a direct air flow path across the wafers. A consideration for achieving in this PLI system a direct air-flow path that passes through a wafer carrier and across the wafers is that reducing floor area occupied by the PLI system correspondingly constrains horizontal depth directed away from the bulkhead opening that may be allotted to the vertical portion of the plenum chamber.
An ULPA filter, that is located directly below the loading platform which receives the SMIF pod, covers an inlet opening in a floor of the bottom portion of the plenum chamber. Ultra-clean air enters the plenum chamber through the inlet opening after passing through the ULPA filter.
In an attempt to establish a uniform horizontal velocity for air flowing across all the wafers throughout the height of the wafer carrier, the prior art PLI system includes a vertically oriented membrane which covers the aperture through which air leaves the vertical plenum chamber to flow horizontally across the wafers. The membrane, which is formed from two layers of GORTEX.RTM. fabric, seals that aperture and separates the vertical portion of the plenum chamber from the area on the loading platform which receives the SMIF pod. In this PLI system, the use of the GORTEX fabric membrane yields a velocity for air flowing across the wafers that varies vertically along the height of the SMIF pod's wafer carrier, and that ranges between 20 and 70 feet/minute. Within the semiconductor processing industry, an accepted standard for air flow is that it must have a uniform velocity both in a specified direction across the wafers, and that the velocity be approximately 90 feet/minute. Accordingly, to more effectively purge contaminants from the PLI system's micro-environment, it is desirable that the air flow across the wafers be increased above that attainable with the two layers of GORTEX fabric, and that air flow uniformity be increased throughout the height of the SMIF pod's wafer carrier.
Because the loading platform which receives the SMIF pod moves up and down, the prior art PLI system includes a flexible bellows that extends downward from below the ULPA filter to a base of the PLI system where the bellows is coupled to a stationary fan. The flexible bellows permits the loading platform to move vertically up and down while communicating a flow of clean air from the fan to the ULPA filter. The flexible bellows appears to be a significant source of contaminants in air flowing across the semiconductor wafers within the wafer carrier.